Large area solid state x-ray detectors are currently being developed in the x-ray art. Such a detector typically comprises a scintillating layer in contact with an array of photodiodes, each with an associated FET switch. The photodiodes are initially charged by connecting them to a known stable voltage by activating the FET switches. Subsequently, the photodiodes are isolated by turning off the FET switches. Upon exposure to x-rays, the scintillator produces light which discharges each photodiode in proportion to the x-ray exposure at the position of the diode. The diodes are then recharged by again connecting them to the known stable voltage. The charge used to restore the diode to its initial voltage is measured by a sensing circuit, and the value is digitized and stored.
In such a detector, the photodiodes and their associated FET switches are organized in rows and columns. The gates of the FETs along a row are connected together, and the row electrodes are connected to scanning electronics. During readout of the detector, rows of FETs are turned on sequentially, and an entire row of detector elements is read out at once. Because of imperfections in the FET switches, a time-dependent background current is generated when the FET switches are turned on and off. This results in an offset signal that is unrelated to the x-ray exposure. Because the rows are read out sequentially, a portion of this offset signal is row-correlated, i.e., it is the same for all elements in a row, but varies from row to row.
In the absence of offsets, the converting circuit would require only the range and resolution of signals generated by an x-ray exposure. In practice, the range of the offsets can be larger than the range of useful signals. For practical reasons, converting circuits have limitations in input signal range, conversion resolution, and conversion speed. In the absence of compensation for offsets, the converting circuit would be required to accommodate an increased input range without sacrificing resolution or speed.
It would be desirable, then, to have a means for compensating for row variable offsets in a large area solid state x-ray detector.